First, you should neuter him. Adult intact males can be very smelly and sometimes down right nasty, also they tend to be meaner than neutered ferrets.
As for the litter training, try buying a different litter pan. There is one, I think it is called the "long john" litter pan, it has clips that hook it to the corner of the cage so he cannot move it.
A few of my ferrets refused to use any litter pan inside their cages, so I put wood pellets all over the entire bottom of the cage.
You can still litter train him for outside play even if he is not trained inside the cage. Ferrets almost always have to use the bathroom when they wake up, I would pick them up and put them in my cat box (which was placed in the corner of the room for them) and did not let him leave until he went (ferrets also tend to pee and poop at the same time). After doing that everytime they first got let out and everytime you see him back up into a corner and then grabbing him and putting him the litter box they start to get the hint and are litter trained.
For added help, put a litter box in each corner of your play room and just keep and eye on him and put him in everytime you see him backing up. He will catch on.
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1 Rat: (Ace)
1 RC Hamster: (Hannibal)
3 Cats: (Tao, Drew, Jak-ke)
2 Ferrets: (Fisher, Georgie)
1 Betta: (Maxwell) + (5 Platies and 1 Molly)
Rest in peace all my little Mice: Tempo, Aaron, Tatiana, Pei, Shi, Orange, Spotty, Lee Hyun, Key, Calvin, Chun, Jiro, Hero...
And my past Dookers: Lulu (my heart), Frodo (my old man), MoneyPennies (the first one), ect...
Aswell as all my lost Cats (my beloved Haru, ect..) and Bettas (Stahl the creepy one, ect..)